Adjust oven rack to lower-middle position and preheat oven to 400°F. Line a baking sheet with parchment paper.: You will notice the oven warming and a steady hum as it comes up to 400°F , that steady heat creates even browning on the bottoms and edges. Placing the rack in the lower middle position encourages a gentle rise while avoiding overly dark tops, and the parchment paper prevents sticking and promotes an easy release. A common mistake is skipping the preheat, which yields uneven bake and dense centers, so always wait until the oven reaches temperature before sliding the tray in.
In a medium bowl, combine sugar and lemon zest; mix with your fingertips until the sugar is moistened and fragrant. Add in the flour, baking powder, baking soda and salt and mix until combined.: As you rub the zest of 3 medium lemons into the granulated sugar , inhale the citrus oils that release, a bright aroma that tells you the flavor is developing. When you fold in the flour , baking powder , baking soda , and salt , the dry mixture should look uniform and slightly grainy. This even distribution ensures consistent rise and flavor in each scone. Avoid overworking at this stage, since overmixing can activate gluten and make the final scones tough.
Grate butter into flour mixture on the large holes of a box grater; use your fingers to work in the butter until the mixture resembles coarse crumbs.: Grating the frozen unsalted butter creates tiny, cold pieces that remain distinct in the dough, and when baked they steam and form flaky layers. As you rub the butter into the dry mix, aim for pea sized bits and a crumbly texture that still has some visible butter. If the butter warms too much and becomes greasy, the dough will lose lift, so work briskly and keep your hands cool. A frequent error is using softened butter, which yields dense, greasy scones rather than flaky ones.
In a small bowl, whisk the sour cream, lemon juice, egg and vanilla until smooth.: Whisking the sour cream , lemon juice , egg , and vanilla extract creates a glossy, slightly thick batter that will hydrate the dry ingredients. The mixture should look homogeneous, and you may see tiny air bubbles form from whisking, which help lightness. This wet mix balances richness and acidity, aiding tender crumb formation. If the wet ingredients are too cold or too warm it can affect integration, so aim for a cool room temperature blend to combine smoothly.
Using a fork, stir sour cream mixture into flour mixture until large dough clumps form. Use your hands to press the dough against the bowl into a ball. The dough will be sticky first, but as you press, the dough will come together.: At this phase you will feel a change from loose crumbs to cohesive, tacky dough. Start with a fork to gently bring things together, then switch to your hands to press and gather into a ball. The tactile feedback is key, you should feel stickiness that resolves as the sour cream hydrates the flour . If the dough feels dry, you may have mismeasured the flour ; resist adding extra liquid, instead press firmly to unify. Overworking the dough here can toughen it, so be deliberate and gentle when forming the ball.
Place on a lightly floured surface and pat into a 7-inch circle about 3/4-inch thick. Use a sharp knife to cut into 8 triangles. Place on prepared baking sheet, about 1 inch apart. Bake until golden, about 15 to 16 minutes. Cool for 10 minutes and prepare the glaze.: Patting the dough into a 7 inch circle gives uniform pieces that bake consistently, and the 3/4 inch thickness ensures a tender center without underbaking. As you cut the triangles, you may feel slight resistance where the butter pockets are, and the surface will have a faint dusting from the flour . Arrange them roughly 1 inch apart so edges brown but do not fuse. During baking you will notice a warm, citrus aroma and edges turning golden; this visual cue indicates they are ready. A frequent misstep is making the circle too thin, which yields dry scones, so keep that 3/4 inch thickness for best results.
In a medium bowl, prepare the glaze by mixing together the melted butter, powdered sugar, vanilla and lemon juice. Whisk until smooth. Dip the top of the scones into the glaze and allow the glaze to harden.: As you whisk the melted unsalted butter with powdered sugar , vanilla , and additional lemon juice , you will see a glossy emulsion form that thins to a dippable consistency. Dip the warm but not piping hot scones into the glaze so it adheres and sets with a delicate sheen. The glaze should harden to a slight snap, creating a pleasant contrast to the tender crumb. Avoid making the glaze too thin or dipping when the scones are steaming hot, or the glaze may slide off; let them cool about 10 minutes first.